1. Field of the Invention
The present invention relates generally to cutting or machining systems for use in severing a device comprising multiple layers of soft and/or flexible materials into individual pieces and particularly to a method and apparatus for slicing thin sheets of laminated soft materials, including, for example, polymer lithium-ion battery laminates, as part of the fabrication of electronic devices, such as batteries.
2. Description of the Prior Art
Because the use of wireless and cellular telephones, laptop computers, portable radios, and other portable electronic products continues to increase dramatically, the need for lighter, smaller, and cheaper power sources continues to be a key priority in the mobile electronics industry. Specifically, it is desirable that batteries be produced having high energy density, i.e. high power to weight ratio, that are also thin and preferably flexible to facilitate powering thin electronic products, such as portable computers, hand-held calculators, and the like. These batteries generally comprise at least two electrode layers (a cathode and an anode) separated by a separator layer, such as a dielectric material, that is nonconductive to electrons, i.e., an electric insulator. To conduct electricity from the electrode layers of the battery to contacts or battery terminals that can be wired to the electronic product being powered by the battery, a metal grid or foil (e.g., aluminum or copper) may be placed adjacent each electrode layer. Manufacturers make these compact batteries from a variety of materials, including nickel cadmium (NiCD), nickel metal hydride (NiMH), and liquid lithium-ion (Li), in an attempt to meet the demands of the mobile electronics industry for smaller, yet more powerful, batteries. However, the NiMH batteries are relatively heavy and inflexible, and cadmium is a tightly regulated substance due to its high toxicity. While liquid lithium-ion batteries provide two to three times the energy density and are, therefore, lighter and smaller than NiMH batteries, liquid lithium-ion batteries still have limited capacity per weight due to heavy packaging design requirements. Because liquid lithium-ion batteries use non-bonded cell technology similar to most other batteries, these batteries require high-modulus, mechanically strong packaging to accommodate pressures applied on the cells or batteries to maintain intimate contact among cell components. The pressures applied on the cells limit the design of such batteries to small cylinder or prismatic shapes to control packaging deformation. However, the cylinder and prismatic shaped cells generally cannot be arranged into multi-cell battery packs without a loss of 20 to 30 percent of total battery volume because of packing inefficiencies. Therefore, it is highly desirable to replace non-bonded cells with bonded, flat plate lithium ion cells to increase the power capacity to weight ratio, enhance cell packing efficiency, and improve cell and battery pack design flexibility.
Since about 1990, the electronics industry has been developing bonded, solid state, flat plate lithium-ion batteries and other polymer batteries. In this battery technology, the polymers used in the three main components (i.e., the cathode, the separator, and the anode) of each battery have similar material properties that enable the entire battery to be bonded together to form a single, laminated unit or sheet. For example, the active material in the anode layer(s) may be carbon or other materials, the active material in the cathode layer(s) may be lithium cobalt oxide (LiCoO.sub.2), lithium manganese oxide (LiMn.sub.2 O.sub.4), lithium nickel oxide (LiNiO.sub.2), and the like. Because the polymer matrix used in the battery is passive to the operation of the battery, any electrically active materials or electrolytes can be utilized in the battery. Polymer lithium-ion batteries may exhibit 20 to 30 percent higher specific energy densities over liquid lithium-ion batteries. Furthermore, polymer lithium-ion batteries may be formed into small, lightweight, flat (i.e., 0.1 to 1 mm thicknesses) shapes that are also flexible and moldable to facilitate installation in myriad electronic product packaging cases or applications. The development of polymer lithium-ion batteries is a significant step toward meeting power, weight, and size goals of the mobile and micro electronics industry, because these batteries may be installed in very thin, compact electronic products having strict overall package sizing requirements. Polymer lithium-ion batteries are also more environmentally benign and are safer to use than many other marketed batteries.
Unfortunately, a continuing impediment to commercial implementation of lithium-ion polymer batteries is the difficulty in manufacturing the batteries effectively and inexpensively on a large, mass production scale. Present manufacturing technologies, such as thin film casting, extrusion, and the like, are readily available to prepare large area, thin (i.e., 0.1 to 1 mm) component layers used in polymer lithium-ion batteries, such as cathode layers, separator layers, anode layers, and conductive foils and grids, which can be bonded easily into a single, large area, laminated unit or sheet under controlled temperatures and pressures. However, cutting or slicing the bonded unit or laminated sheet of relatively soft polymer battery material into smaller pieces for practical applications continues to present major challenges to battery manufacturers. Current cutting methods generally utilize cutting wheels with sharp, knifelike edges to cut through the large area, multilayer sheets to slice them into smaller, more useable sizes. A downward, compressive force is applied to the cutting wheel to push the sharp edge of the cutting wheel transversely through all the layers of the large area, laminated sheet while the cutting wheel rotates passively as it is moved laterally in relation to the sheet. Such rotation of the cutting wheel reduces lateral frictional forces developed in the laminated sheet that would tend to distort material layers in a lateral direction during cutting. However, due to the softness and flexibility of the polymer, metal foil, and other layers of the sheet, the sharp edge of the cutting wheel still compresses, distorts, and pinches the layers of material together at the cut surfaces as the edge of the cutting wheel is pushed transversely through the layers of the sheet. Such pinching and deformation can and often does result in the anode and cathode layers and/or the top and bottom metal grids or conductor layers coming into contact or at least into very close proximity to one another, which short circuits the anodes and cathodes of the separated sheets and renders them useless as batteries.
Because the industry has not overcome these short circuit problems caused by such state-of-the-art slicing apparatus and processes, manufacturers have been forced to resort to more labor and equipment-intensive manufacturing methods to produce lithium-ion polymer batteries, producing each battery individually, instead of in mass, by fabricating each layer of the battery to its final dimensions and then aligning and assembling or laminating each layer together to form the desired multilayer battery device. This kind of process and other inefficient alternate manufacturing methods have hindered large-scale production of polymer lithium-ion batteries.
Consequently, there remains an acute need for manufacturing methods and apparatus that enable cutting or slicing of large area sheets of soft, flexible, multiple layer materials into desired shapes and sizes without distortion at the layers at the cut edges to facilitate large-scale commercial implementation of the smaller, more flexible, and higher energy density electronic devices, such as batteries. Specifically, an improved slicing method and apparatus is needed to eliminate the problems of short circuiting layers of electrically-conductive polymer and/or metal foil materials used in lithium-ion polymer batteries in an effective and reliable, yet inexpensive manner.